Two men dressed in camouflage stand on a patch of dirt amid rubble, Kalashnikov rifles at their sides, at the entrance to a dark hole in the dirt. The wooden screens often found in mosques lay on the ground, cast aside. The hole was the burial place of Hujr bin Uday al-Kindi, one of the prophet Mohammad's companions, widely revered by Muslims, Shiites in particular.

The men standing on top of it are members of Jabhat al-Nusra, a Sunni Muslim extremist rebel group trying to topple the regime of President Bashar al-Assad that recently swore fealty to al Qaeda's leader, Ayman al-Zawahiri.

The desecration of the shrine - and the removal of the remains - drew condemnation from the highest levels of Shiite Islam. Iran's Supreme Leader Ali Khamenei called it "bitter and sad," while the militant group Hezbollah in Lebanon issued a statement stark warning that it "foretells a large conflict and gloomy evil."

That large conflict is what many fear is the next chapter of Syria's brutal two-year war. As it drags on, it has grown more sectarian and more likely to spill over into neighboring countries. In the immediate term, Lebanon would be foremost among them, directly pitting Hezbollah militants against al Qaeda-linked jihadists who have flocked to Syria from across the Muslim world.

Hezbollah fighters are already fighting those rebels on a relatively small scale in Syria, the group's leader confirmed last week. Several dozen are believed to have been killed in the past several months, their bodies sent back to Lebanon for burial.

Most of the action they've seen recently has been defending the Lebanese Shiites living in over 20 border villages inside Syria, notably al-Qasr, home to Lebanese Shiites and Christians which has come under attack by fighters from Jabhat al-Nusra.

The second of Hezbollah's main missions in Syria is to defend the Sayyida Zeinab shrine near Damascus (other Shiites - Iraqis - are believed to be there as well). The ornately decorated shrine is where the granddaughter of the prophet Mohammed is buried, a highly sacred place for Shiites that normally sees pilgrims visiting year-round.

In a conflict in which the phrase "red line" has been bandied about by most of its participants, Hezbollah's leader, Shiekh Sayyed Hasan Nasrallah, last week seemed to draw one of his own, warning of "serious repercussions" if the shrine is attacked.

Syria's conflict is often described as a "civil war," but that is only true insofar as it has yet to spill over into another country on a large scale or draw in too many different forces. But it is the quintessential proxy war, with the Alawite (an offshoot of Shia Islam) Assad regime backed up by Shia allies Hezbollah and Iran, as well as Russia and China.

The Sunni rebels are supported by the Islamist rulers of Saudi Arabia, Qatar and Turkey, as well as the U.S., France, Britain and others.

Raising tensions further are the twin air strikes carried out by Israel in Syria last on Friday and Sunday, the primary targets are believed to have been missiles heading to Hezbollah from Iran, their primary backer.

"Iran stands at the side of Syria in the face of Israeli aggression, whose aim is to damage the security of the region and weaken the axis of resistance," Iranian Foreign Minister Akhbar Salehi told Assad in a meeting in Damascus on Tuesday.

Today, Hezbollah is believed to have around 60,000 missiles, capable of hitting every part Israel. The missiles hit by Israel are believed to have been the highly-accurate Iranian-made Fatah-110s. In a televised speech on Wednesday night, Nasrallah said Syria would still give Hezbollah "game-changing weapons it has not had before."

"In the whole of Arab history, no other Arab regime has given us as much as President Bashar al-Assad's regime has," Nasrallah said.

"Those who want an immediate response should look in the direction of Iran and Hezbollah," wrote the editor-in-chief of Lebanon's Al Akhbar newspaper, Ibrahim al-Amin, on Thursday. "There are enough indicators on that front to suggest that there is no escaping some kind of reply."

Syria's conflict is often described as a "civil war," but that is only true insofar as it has yet to spill over into another country on a large scale or draw in too many different forces. But it is the quintessential proxy war, with the Alawite (an offshoot of Shia Islam) Assad regime backed up by Shia allies Hezbollah and Iran, as well as Russia and China. The Sunni rebels are supported by the Islamist rulers of Saudi Arabia, Qatar and Turkey, as well as the U.S., France, Britain and others.

As the fighting grows among the regime, its ally Hezbollah and the Sunni jihadists, the U.S. had treaded cautiously to avoid inflaming the conflict to the point of boiling over. But the spillover violence could force their hand.

"America could get sucked into this because they have alliances with all of Syria's neighbors," said Tabler. "They don't want to help either side but that fighting is going to destroy Syria as we know it today."

In December, Syrian rebels burned down a Shiite mosque in northern Idlib province. Fighting between Hezbollah and Jabhat al-Nusra is being waged closer and closer to the Zeinab shrine. Shiite villages are coming under attack by militants who praise Osama bin Laden and Sunni villagers are being slaughtered by regime loyalists. Sectarian fighting has already leaked across the border into northern Lebanon. The stage has been set.

"When Hezbollah and Israel are both actively fighting in the same third country," writes Ramy Khoury, a professor of international affairs at the American University of Beirut, "and Iran and the United States are both actively warning about their determination to act to protect their allies and their interests in that same third country, it is time to make another pot of coffee and make sure you have plenty of fresh batteries at home for your transistor radio."